There are currently on the market a number of appliances with different configurations for forming lock stitches in order to produce a seam in a piece of material being sewn, said piece of material hereinafter referred to for the sake of simplicity as fabric. Ordinary domestic sewing machines conventionally involve the use of an upper thread and a lower thread on a bobbin which in cooperation with a needle causes the upper thread to execute a lock stitch in the fabric being sewn in the sewing machine.
Sewing machines of the lock stitch type have since a long time been part of the state of the art and their mode of operation is well-known. Taking, for the sake of simplicity, a sewing machine with a single needle as an example, such a machine forms stitches by the upper thread and the lower thread being linked together by the needle moving to and fro through a fabric which is moved forward across a sewing table, which is usually in a plane substantially perpendicular to the length of the needle. Most conventional sewing machines of this kind have a take-up lever which pulls the upper thread from an upper thread storage bobbin. The take-up lever provides the needle with the upper thread by an oscillating movement towards and away from the fabric. The expression “upper” hereinafter means the side of the fabric where the needle is housed. “lower” means the side of the fabric where the making of a knot is effected. Also, the expression “thread” hereinafter always means “upper thread” unless otherwise indicated.
When the take-up lever is in its highest position, a maximum amount of thread has been drawn out for the next stitch, after which the movement of the take-up lever reverses back downwards. After the take-up lever's reversal, the thread will form a loop under the fabric, since the effect of friction in the fabric will result in not all of the thread drawn out being immediately drawn back by the take-up lever.
The lower thread is unwound from a lower bobbin accommodated in a gripper under the fabric. The gripper may be of a rotating type and equipped with a gripper tip (sometimes called gripper arm) which in the course of the gripper's rotary movement hooks into the loop formed by the upper thread and in its continuing movement leads the upper thread round the lower bobbin.
When the oscillating movement of the take-up lever takes it upwards away from the fabric, the take-up lever draws surplus upper thread back, i.e. the amount of thread not consumed in the respective stitch. The thread drawn forward constituting said loop will thus be pulled tight so that a lock stitch is formed by the upper thread and the lower thread in cooperation, since the gripper has led the upper thread round the lower thread. A feeder on the sewing machine will then move the fabric forward for a subsequent stitch.
Said oscillating movements executed by the needle, the take-up lever and the gripper are mutually synchronised and are repeated cyclically for each stitch executed with the sewing machine.
Generally, a gripper system is nowadays so configured that the gripper rotates about a lower thread bobbin. A distinction may be made among gripper systems of two types, one of them with the gripper rotating in the horizontal plane, the other with the gripper rotating in the vertical plane, parallel with the needle. To achieve advantages with horizontal grippers, they have to be situated ahead of the needle, which makes it easier to reach a lower thread bobbin case in order to change the bobbin or the thread on it without having to remove the fabric from the sewing table. The gripper system used in the sewing machines referred to in this application has a horizontally disposed gripper. The gripper is provided with a bobbin basket, in which the lower thread bobbin is placed. During sewing, the bobbin remains stationary while the gripper rotates about it.
The needle directing the upper thread is fitted to the bottom of a needle rod which, synchronously with the other parts of the sewing machine involved in forming a stitch, moves the needle up and down in an oscillating movement. The needle is also allowed to move sideways synchronously with the formation of stitches. Sideways movement of the needle is necessary in the case of stitches required to have a width, i.e. to have the thread move a distance sideways across the fabric during sewing. This involves a difficulty in the case of a horizontally fitted lower bobbin, since the needle has to be adjacent to the gripper when said loop is formed if a safe capture of the loop should be rendered possible for the gripper. Accordingly, the sideways movement of the needle in the horizontal plane has to follow a slightly curved path adapted to the radius of the gripper in its rotary movement. This is readily observable on a domestic sewing machine, where a stitch plate serving as support for the fabric and at the same time covering the lower bobbin space is provided with and discloses a needle hole with a curved path, whereas the corresponding path on a sewing machine with a vertical lower bobbin is straight.
The mechanical components of the sewing machine with a horizontal lower bobbin are so arranged that the needle rod describes a movement along a conical surface, which movement is synchronised with that of the gripper. The technology for the movement of the needle in this context is known and is not further discussed here.
An example of prior art technology for a sewing machine of the type discussed herein is, as an example, described in U.S. Pat. No. 4,432,293, the content of which is hereby in its entirety incorporated in the present description.
As a consequence of the aforesaid curved path which the needle follows in the horizontal plane, it is clearly observable, on a finished stitch which has a width, that the thread follows a curved path if the sideways deviation is sufficient.